Knowledge of wellbore placement and surveying is useful for the development of subsurface oil & gas deposits. Directional borehole drilling typically relies on one or more directional devices such as bent subs and rotary steering systems to direct the course of the wellbore. The angle between the reference direction of the directional device and an external reference direction is referred to as the toolface angle, and determines the direction of deviation of the wellbore. Directional drilling proceeds through comparing the placement of the borehole with the desired path, and selecting a toolface angle and other drilling parameters to advance the borehole and correct it towards the planned path. Measurement of toolface thus may be a component for borehole steering and placement.
When determining toolface, an external reference direction for the toolface may be chosen based on the geometry and location of the wellbore. In deviated wellbores, with an inclination away from vertical in excess of 5-8°, the usual reference is the direction of acceleration due to gravity. This may be measurable via accelerometers which rotate with the drill string, such as during measurement while drilling (MWD). In a vertical well or near-vertical well, the direction of gravity may be aligned or substantially aligned with the drill string axis and may not be able to provide a useful reference direction. Several alternatives may be used in place of accelerometers in vertical or near-vertical wells. Traditionally, magnetic toolface may be used, which applies the onboard magnetometers used in MWD to use the Earth's magnetic field as a reference direction. However, magnetic toolface may fail at sufficiently high magnetic latitude, or where magnetic interference from nearby wellbores, surface facilities, or other effects alter the local magnetic field. Another alternative for a reference is the true North available from a north-seeking downhole gyroscope, or a reference carried down by a non-north-seeking gyroscope. Gyroscopes may suffer from cost and reliability concerns.